Macrophages, T and B cells,
primary and secondary immune organs,
mucosal immune system
Macrophages
 Terminal stage of monocyte-macrophage line
differentiation
 Monocyte-macrophage cells differentiate from
myeloid precursor (developed from pluripotent stem
cell bearing CD34) in bone marrow
 Matured monocytes are released to peripheral blood
stream, then move in organs and develop into tissue
macrophages
Development
of monocytes and macrophages is affected by various
cytokines:
 SCF(stem cell factor): produced by stromal cells
→ activation of stem cell
 GM-CSF (granulocyte-monocyte colony stimulating
factor): produced by bone marrow (BM) stromal cells,
lymphocytes → stimulation of monocyte production
 M-CSF (monocyte colony stimulating factor):
produced by stromal cells, lymphocytes, endothelial
and epithelial cells → production and maturation of
monocytes
 IL-3: produced by lymphocytes → production of
monocytes (and other blood cells)
Macrophages- development
 Monocytes- in the blood (7%) and the rest in
bone marrow
 Macrophages - in tissues
histiocytes
Macrophages
 a monocyte enter damaged tissue through the
endothelium of a blood vessel
 a monocyte is attracted to damaged site by chemokines,
triggered by stimuli including damaged cells, pathogens
and cytokines released by macrophages
 after migration of monocytes to the tissues, they
differentiate into different forms of macrophages
 macrophages survive several months
Macrophage surface molecules
 MHC gp I, II assist in the presentation of
antigen to T lymphocytes
 CD 35 - complement receptor 1 (CR 1),
binds complement C3b
 Receptor for the Fc portion of IgG
 CD 14 - receptor for bacterial
lipopolysaccharides
Cytokines produced by macrophages
 IL- 1 α, ß - stimulate both T and B cells, Ig synthesis,
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activation of other macrophages, sensitizing cells to IL-2
and IFN
TNF- α - similar in function to IL-1
IL- 8 - secreted by activated macrophages
- chemokine attracting neutrophils and T cells
IL-12 - promotes induction of Th1 cells, inhibits Th2
cells
IFN- α- activates host cells to induce enzymes inhibiting
viral replication; increases expression of MHC gp I on
host cells; activates NK cells, T cells, other
macrophages
Functions of macrophages
 Phagocytosis
 Production of cytokines
 Presentation of epitops with MHC gp II
 Presentation of epitops with MHC gp I
Phagocytosis
 a foreign substances are ingested
 microbes are killed and digested
 follows processing of antigenic epitopes and
their presentation on the cell membrane
Macrophage - functions
 Macrophages provide defense against tumor cells
and human cells infected with fungi or parasites.
 T cell becomes an activated effector cell after
recognition of an antigen on the surface of the APC
→ release chemical mediators → stimulation of
macrophages
Presentation of epitopes with
MHC gp II
 After endocytosis and degradation of the antigen,
presentation of its epitopes follows
 epitope is connected to MHC gp II → cell surface →
presentation to Th cells
 MHC (Major Histocompatibility Complex) = complex
of genes that governs the production of the major
histocompatibility antigens - in humans termed HLAs
(Human Leukocyte Antigens)
Presentation epitopes with MHC
gp I
 intracellular parasites are hydrolyzed in
proteasomes of macrophages
 their peptides are connected to TAP (Transporters
Associated with antigen Processing molecules 1,2),
that carry the epitope and MHC gp I → presentation
on the cell surface to Tc cells
Antigen presentation
Dendritic Cells (DC)
 DC mature after a contact with pathogen, then migrate to
lymph nodes where antigen-specific immune response
develops
 DC are equipped with numerous cytoplasmic processes,
allowing contact with up to 3000 T cells
 In lymph nodes, the expression of MHC gp I and co-
stimulatory molecules (CD80, CD86) on DC increases
Types of Dendritic Cells
 Myeloid DC
– similar to monocytes
– give rise to Langerhans cells (epidermis), interticial DC
(lymph nodes)
 Lymphoid DC
– give rise to plasmocytoid DC - looks like plasma cells,
but have certain characteristics similar to myeloid cells,
they produce huge amounts of interferons
Function of DCs
 DCs are the most important APC
 DCs can be easily infected by viruses → processing of
viral proteins → their presentation in complex with MHC
gp I → activation of Tc
 DCs can ingest extracellular viral particles → their
presentation in complex with MHC gp II → activation of
Th2 cells → help for B cells → production of antiviral
antibodies
 DCs can also be activated by apoptotic cells
Antigen Presenting Cells (APC)
 Dendritic cells, macrophages, B cells
 Antigen processing and its presentation to
T cells in the complex with HLA class I or II
 Providing additional signals to T cells which
are necessary for their activation (CD 80, CD
86)
T cells: ontogenesis, surface markers.
Subpopulations of T cells and their functions.
T lymphocytes - ontogenesis
 Stem cell in BM gives rise to lymphoid precursor cell
which matures into 3 types of lymphocytes:
 T lymphocytes
 B lymphocytes
 Natural killer (NK) cells
 Pro-thymocytes move to the thymus where continue
the maturation into T lymphocytes
 Maturation of B lymphocytes continues in BM
Surface markers of T cells
 CD (Cluster of Differentiation) proteins -
molecules on the cells membrane, they allow
the identification of cells
 TCR - receptor for antigen
 MHC gp I
CD proteins
 allow an identification of T-cell subsets
 CD 2 = adhesion molecule
 CD 3 = important in intracellular signaling (initiation of
immune response); closely associated with TCR
 CD 5,7
 CD 4,8 = are expresed on subclasses of mature T
cells; CD4 reacts with MHC gp II,CD8 reacts with
MHC gp I on macrophages
 CD 28 – molecule that provides co-stimulatory
signals, binds CD80 and 86
Maturation of T lymphocytes
Consist of three types of processes:
 Proliferation of immature cells
 Expression of antigen receptors genes
 Selection of lymphocytes
TCR
 Antigen receptors are encoded by several gene
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segments that recombine during lymphocyte maturation
Heterodimer consisting of 2 nonidentical polypeptide
chains linked together by disulfide bonds
> 95% T cells express the αß heterodimer, 5% γδ
TCR heterodimer is noncovalently associated with the
γ,δ,ε chains of the CD3 molecule
complex TCR-CD3 makes contact with both the Ag and
MHC gp
Subpopulation of T cells
 Subpopulation of T cells are defined according to
their particular function and their CD membrane
markers
 T cytotoxic cells (Tc) CD8+
- recognize the foreign epitope in association
with class I MHC molecules
 T helper cells (Th) CD4+
- recognize the epitopes in association with
class II MHC molecules
T cytotoxic lymphocytes (Tc;CD8+)
 cause lysis of target cell; active against tumors, virus-
infected cells, transplanted allogeneic tissue
 release TNF → decrease of proteosynthesis
 recognize the foreign epitope in association with
MHC gp I molecules
 Destroy target cells by perforins (create pores in the
cell membrane → cell lysis) and granzymes
(degradation of essential macromolecules)
T helper lymphocytes
(Th; CD4+)
 recognize the epitopes in association with
MHC gp II
 help for B cells to produce antibodies and
help for phagocytes to destroy ingested
microbes
 subsets of Th cells: Th1, Th2 cells
Regulatory T cells
 Express CD4, CD25, FoxP3
 Regulate the activation or effector function of
other T cells
 Are necessary to maintain tolerance to self
antigens
 Production of IL-10, TGF-b
The role of thymus.
Positive and negative selection
of T lymphocytes.
The role of thymus
 In thymus, lymphocyte precursors from the
bone marrow become thymocytes, and
subsequently mature into T cells
 Once matured, T cells migrate from the
thymus and constitute the peripheral T cell
repertoire responsible for specific cell
response
Phases of thymocyte maturation
 A rare population of hematopoietic progenitors enters
the thymus from the blood, and expands to a large
population of immature thymocytes
 Immature thymocytes each produce distinct T cell
receptors by a process of gene rearrangement.
 This process is error-prone, and some thymocytes
fail to make functional T cell receptors, whereas other
thymocytes make T cell receptors that are
autoreactive
Positive and negative selection
 Immature thymocytes undergo a process of
selection, based on the specificity of their T
cell receptors.
 This involves selection of T cells that are
functional (positive selection), and
elimination of T cells that are autoreactive
(negative selection)
Positive selection of T cells
Entrance of precursor T cells into thymus from the
blood
2. Presentation of self-antigens in complexes with
MHC molecules on the surface of cortical epithelial
cells to thymocytes
3. Only those thymocytes which bind the MHC/antigen
complex with adequate affinity will receive a vital
"survival signal"
4. The other thymocytes die (>95%)
1.
Negative selection of T cells
1. Thymocytes that survive negative selection migrate
towards the thymic cortex and medulla
2. Presentation of self-antigen in complex with MHC
molecules on antigen-presenting cells
3. Thymocytes that react inappropriately strongly
with the antigen receive an signal of apoptosis
B-lymphocytes - ontogenesis, surface
markers, function.
B-lymphocytes
are an essential component of the adaptive immune
system
 Maturation of B cells takes place in BM
 B cell originates from stem cell and need to be in
touch with the stromal cells in the bone marrow
 Stromal cells produce SCF (stem cell factor)
necessary for development at early period, IL-7
necessary at later period of maturation
 Ig gene rearrangements and the appearance of
surface markers identify the stage of B-cell
development
Development of B lymphocytes
 Lymphoid progenitor → pro-B cells
 During maturation from pro-B cells into pre-B cells: Ig
genes of the heavy chain recombine; pre-B cells express
pre-BCR
 During maturation from pre-B cells into B cells: Ig genes
of the light chain recombine
 Immature B cells express membrane IgM
 Mature B cells express membrane IgM and IgD = BCR
and are able to respond to antigen in peripheral
lymphoid tissues
Negative selection
 If an immature B cell binds an antigen in the bone
marrow with high affinity → further maturation is
stopped and B cell dies by apoptosis
 Negative selection eliminates potentially dangerous
cells that can recognize and react against self
antigens
 B cells that survive this selection process leave the
bone marrow through efferent blood vessels
B-lymphocytes – surface markers
 CD 10 - immature B cells, malignant cells
 CD 35 - receptor for the C3b of the
complement
 CD 19 - characteristic marker of B cells
 CD 20 - typical surface antigen of Ig-positive
B lymphocytes
 IgM, IgD - antigen receptors = BCR
 MHC gp II - antigen-presenting molecules
B-lymphocytes – functions
 After stimulation B lymfocytes convert into the
plasma cells and produce antibodies
against soluble antigens
 Other functions are :
antigen presentation
cooperation with complement
system
Primary immune organs and their role in
the immune system.
Primary immune organs
 Bone marrow
 Thymus
 are organs of development, differentiation and
maturation of immune cells and elimination of
autoreactive cells
 T and B lymphocytes mature and become
competent to respond to antigens in PIOs
Bone marrow
is the central cavity of bone and the site of
generation of all circulating blood cells in adults,
including immature lymphocytes, and the site of Bcell maturation.
 The pluripotent stem cell gives rise to the
progenitors of all immune cells
 Production of the cells takes place in the spaces
divided by vascular sinuses
 Endothelial cells of the sinuses produce
cytokines
 Sinuses are bordered by reticular cells
Differentiation in the BM
 Differentiation from the stem cell is influenced
by:
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membrane interaction between the stem
cells and the stromal cells
cytokines (CSF, IL-3, thrombopoetin,
erythropoetin)
Thymus
 is located between the sternum and
the major vessel trunks
 It consist of two lobes
 Each lobe is surrounded by a
capsule and is divided into lobules,
which are separated from each other
by strands of connective tissue =
trabeculae
Structure of the thymus
Each lobule is organized into two compartments:
- the cortex (outer compartment) – contains
lymphocytes that proliferate
- the medulla (inner compartment)- mature
lymphocytes, Hassall´s corpuscles
Thymus - morphology
Various kinds of stromal cells:
 thymic epithelial cells – production of thymulin,
thymopoetin, thymosin that influence the maturation
of T cells
 dendritic cells
 macrophages
 The thymus contain a large number of blood vessels
and efferent lymphoid vessels that drain into the
mediastinal lymph nodes
Secondary immune organs - structure and
function of lymphatic node and spleen.
Secondary immune organs
• consist of the spleen, the lymph nodes, the mucosal and
cutaneous immune system
• are organized to optimize interactions of antigens, APCs
and lymphocytes
• are places of the development of adaptive immune
responses
spleen
lymphatic
nodes
tonsils
MALT
appendix
Peyer´s
patches
Lymphatic node
 are nodular aggregates of
lymphoid tissues located along
lymphatic channels throughout
the body
 Lymph comes from tissues and
most parenchymal organs to
the lymph nodes
 Lymph contains a mixture of
substances absorbed from
epithelia and tissues
 As the lymph passes through
lymph nodes, APCs in the LN
are able to sample the
antigens of microbes that may
enter through epithelia into
tissues
Lymphatic node
• lymph circulates to the lymph
node via afferent lymphatic vessels
and drains into the node just
beneath the capsule in a space
called the subcapsular sinus
• the subcapsular sinus drains into
trabecular sinuses and finally into
medullary sinuses
• the sinus space is criss-crossed
by the pseudopods of
macrophages which act to trap
foreign particles and filter the lymph
• the medullary sinuses converge
at the hilum and lymph then leaves
the lymph node via the efferent
lymphatic vessel
Lymphatic node - medulla
• The medullary cords are cords of lymphatic
tissue, and include plasma cells and T cells
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The medullary sinuses are vessel-like spaces
separating the medullary cords; contain
histiocytes (= immobile macrophages) and
reticular cells.
•
Lymph flows to the medullary sinuses from
cortical sinuses, and into efferent lymphatic
vessels
Lymphatic node - cortex
Contains lymphoid follicles = accumulation of Blymphocytes and follicular dendritic cells
When a lymphocyte recognizes an antigen, B cells
become activated and migrate to germinal centers =
to the secondary nodule
Spleen
is a secondary lymphoid organ located high in the left abdominal
cavity
 is surrounded by a capsule, which sends trabeculae into the
interior to form a compartmentalized structure
 there are two types of compartments -red pulp and white pulp
with a marginal zone in between
 is NOT supplied by afferent lymphatics
Spleen
 Red pulp : place of mechanical filtration and
elimination of senescent red and white blood cells
and microbes
 White pulp : T lymphocytes CD4+,CD8+ are around
arterioles (periarteriolar lymphoid sheaths), B
lymphocytes are in the follicles; final maturation of B
lymphocytes course in germinal center of secondary
follicles
Mucosal immune system
 MALT = mucosa-associated lymphoid tissue
 GALT = gut-associated lymphoid tissue
 BALT = bronchus-associated lymphoid tissue
 GIT, respiratory, and urogenital systems are lined by
mucous membranes
 Includes clusters of lymphoid cells in lamina propria
of intestinal villi
 contains a very large population of plasma cells that
synthesize IgA antibodies
M cells
 are epithelial cells that are specialized for the transport antigen from
the lumen of the respiratory, GIT, and urogenital tracts to the
underlying MALT
 contain a characteristic pocket filled with B cells, T cells, and
macrophages
 are found at inductive sites that overlie organized lymphoid follicles
in the lamina propria
 antigens are endocytosed and transported within vesicles from the
luminal membrane to the pocket membrane, where the vesicles fuse
and deliver their contents to antigen-presenting cells
Secretory IgA
 daily production of secretory IgA into mucus
secretions exceeds that of any other class of
immunoglobulin (5-15 g each day)
 is an important line of defense for mucosal surfaces
against bacteria
 binding of secretory IgA to bacteria and viruses also
prevents attachment to mucosal epithelial cells,
thereby inhibiting infection and colonization
Cutaneous immune system
 Epidermis contains keratin cells that produce
IL-1, 6 and TNF during inflammation; and IL10, TGF-β during healing
 Dermis contains fibroblasts that produce
collagen, remove apoptotic cells
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